added get_previous_output to 'output-layout'

[dvrlabs]

Notice: Wayfire's development is temporarily happening in the stabilize-api branch. If you open a pull request for the master branch, it will likely not be merged before the stabilize-api branch is itself merged into master.

Trying to get setup to address #779

[dvrlabs]

Almost have this working on my machine, with the exception that 'next_output' wraps, but previous_output does not.

[dvrlabs]

Think I've got the wrapping working now. Should be done!

[ammen99]

Closing because of staleness. Feel free to re-send (with review comments addressed) if you still want this feature.